Image intensifier with ferroelectric layer and balanced impedances



Dec. 20,1966 B.KAzAN ETAL 3,293,441

IMAGE INTENSIFIER WITH FERROELEGTRIC LAYER AND BALANCED IMPEDANoEs vFiled May 12, 1965 Low/LEVEL RAmATmN PATVERN Zi?, I'

l oco/vac'r/j/f /ff/ekoafcr/e/c v afar/ML M/A/fscf/vr Cpc' www WAGE JOHN5. W//VSLOW INVENTORIS ATTORNEY l United States Patent Mice 3,293,441IMAGE INTENSIFER WITH FERROELECTREC LAYER AND BALANCED MPEDANCESBenjamin Kazan, 2062 Tigertail Road, Los Angeles, Calif.

90049, and .lohn S. Winslow, 4380 Canyon Crest Road,

Altadena, Calif. 90065 Filed May 12, 1965, Ser. No. 456,901 2 Claims.(Cl. Z50- 213) The present invention relates in general to 4lightamplifiers, and more particularly relates to a ferroelectric imageintensifier device.

This application is a continuation-in-part of the original applicationtiled May 6, 1963, and given Serial No. 278,287, now abandoned.

Because of its ability to intensify and store images as well as convertthem from one wavelength to another, combined with its structuralsimplicity and compactness, the two-layer solid-state light amplierstimulated considerable interest when first demonstrated a few yearsago. Such amplifiers were developed to the point where they exhibitedradiant energy gains of several hundred, using visible input light, andwere capable of producing output images of a quality exceeding that ofcommercial 50G-line TV pictures. When operated with X-ray input images,these intensiers produced an output brightness 100 times greater than aconventional uoroscope screen, at the same time greatly increasing theimage contrast. However, devices of this type have a number oflimitations, namely:

(l) Slow speed of response, which interferes with the reviewing ofmoving objects;

(2) Limited resolution because of the photoconductor thickness, which isnecessary to withstand the operating voltages;

(3) Relatively high input radiation threshold;

(4) Inherently high contrast ratio or gamma, thereby limiting itsusefulness in some applications; and

(5) Restriction to the use of photoconductors with extremely highsensitivity and low dark current.

The above limitations result both because of the operating principles ofsuch a two-layer device and the inherent properties of the materialsutilized therein. To be more specic, an example of such a two-layerdevice is disclosed in the patent to William O. Reed entitledPhotosensitive Radiant-Energy Transducers, issued November 26, 1963, andhaving Patent No. 3,112,404. As taught in the patent, Reed uses alaminar structure that includes photoconductive and electroluminescentlayers sandwiched between a pair of transparent electrodes, an A.C.voltage source being connected across the electrodes. However, in thiskind of two-layer device, the A.C. impedance of the photoconductivelayer must be considerably higher than the A.C. impedance of theelectroluminescent layer if the photoconductive layer is to have anycontrol action at all. In other words, unless the capacitance of thephotoconductive layer is very much smaller than that of theelectroluminescent layer, the phosphor or electroluminescent materialwould be emitting light irrespective of the excitation of thephotoconductor, an obviously undesirable characteristic.

This can be explained by schematically representing each of the twolayers with the parallel combination of a capacitor and resistor, thetwo combinations being connected in series between an A.C. voltagesource.

Since the resistance of the unilluminated photoconductive 3,293,441Patented Dec. 20, 1966 layer is extremely high, it will be obvious toanyone skilled in this art that the capacitance of the layer must bevery low as compared to the capacitance of the electroluminescent layer,that is to say, the A.C. impedance of the photoconductive layer must beconsiderably higher than that of the electroluminescent layer, or else amajor portion of the applied voltage would appear across theelectroluminescent layer. This means that unless the proper conditionsexist, a significant amount of electrical current will ow through theelectroluminescent layer, even though the photoconductive layer isunilluminated which, in turn, would cause the phosphor to emit lighteven in the absence of any radiant-energy input.

On the other hand, with the necessary relatively high A.C. impedancepresent in the photoconductive layer, the incidence of light thereoncauses its resistance to drop to a very low value, which, from apractical point of view, shorts or by-passes the A.C. impedance. As aresult, a major portion of the applied A.C. voltage is thereby developedacross the electroluminescent layer and, in response thereto, the iiowof current through this layer materially increases so that there is asharp increase in the amount of light emitted by it.

It is thus seen .that if the Reed device is to function at all, or atleast with any elfectiveness, the photoconductive layer must be made tohave a very much smal-ler capacitance than the adjacentelectroluminescent layer. This is done by making the photoconductivelayer thicker than it might otherwise be.

To a large degree, the limitations enumerated above were overcome by anew solid-state image intensifier approach as taught in the patents toBenjamin Kazan entitled Light Amplifying Device, Patent No. 2,905,830,issued September 22, 1959, and the patent to Richard K. Orthuberentitled Solid-State Radiation Amplilier, Patent No. 3,054,900, issuedSeptember 18, 1962. In this approach, three layers are employedconsisting, respectively, of a photoconductor, a ferroelectric material,and an electroluminescent phosphor. By means of the added ferroelectriclayer, the over-all operation compared to a two-layerphotoconductiveelectroluminescent amplier is improved in many respects.Specifically, the ferroelectric amplifier approach allows thefabrication of image intensifiers with the following advantages:

(1) Increase of radiant-energy gain;

(2) Increase of speed of response;

(3) Increase of output brightness;

Decreaseof input radiation threshold;

Control of input threshold;

Use of D.C. operated photoconductive layers; Control of output imagepolarity;

Control of contrast or gamma; and

Use of photoconductors with increased dark currents.

However, notwithstanding the improvements mentioned, previous designs offerroelectric image intensiiers, as may be seen from the above-saidpatents to Kazan and Orthuber, have all required complex electrodingwith one or more of the layers being broken into elements, segments orlines. This complex electroding and breaking up of the layers is madenecessary by the fact that the ferroelectric material, such as bariumtitanate or triglycine sulfate, has such a high resistivity as comparedto the resistivity of the photoconductive and electroluminescentmaterials that if they were all placed together in a simple three-layerarrangement of the kind suggested by the Reed patent, then there wouldnot be any control action by the photoconductive layer, that is to say,when light irnpinged on the photoconductor material, there would stillnot be any significant change in the voltage or electric eld,

across the ferroelectric and electroluminescent layers. In other words,the change in the electric elds across .the ferroelectric andelectroluminescent layers would be negligible, with the result that thecurrent ow through these layers would likewise change very little. l

Hence, a three-layer laminar arrangement as suggested, for example, bythe combinati-on of the Grthuber and Reed patents, would not workbecause they would be very little fluctuation of the light emitted bythe electroluminescent layer in response to the fluctuations of lightincident upon the photoconductive layer. In other words, in order toproduce the desired amplifying action, the D.C. voltage across theferroelectric layer must change significantly in response to thevariations of conductivity or resistance of the photooonductive layer ifthe A.C. current through the fer-roelectric and electroluminescentlayers is to be modulated to 1a signicant degree. This cannot beachieved simply by combining the materials of Orthuber with thestructural arrangement of Reed. Consequently, to overcome this problem,it has been necessary in the prior a-rt to segmentize at least thephotoconductive layerV and to employ complex electroding as a resultthereof in order'to obtain the desired amplifying action.

The present invention permits the construction of an intensifier usingonly continuous layers of materials without requiring an array ofaccurately-spaced electrodes over the image area. As a result, the basicconstruction of such an image intensifier device is very greatlysimpliiied, higher resolution is obtainable, and improved -uniformitycan be expected. The present invention accomplishes these resultsthrough the alteration of the extremely high resistivity characteristicsof the ferr-oelectric, to suchan extent that the resstances of thephotoconductive, ferroelectric and electroluminescent layers are of thesame order of magnitude, that is to say, comparable to each other invalue, and this is done by doping the ferroelectric material, the termdoping being a term that is Well understood in the art. A furtherbenefit derived from the present invention lies in the fact that thecapacitive impedance of the phot-oconductive layer need not be as greatas in the Reed case, preferably smaller in value than the capacitiveimpedance of the electroluminescent layer, with the result that a muchthinner photoconductive layer is used here than in the prior art.

It is, therefore, an object of the present invention to provide animproved three-layer solid-state light-amplifying device.

It is another object of the present invention to provide a three-layersolid-state image-intensifier device in which only continuous layers areused.

It isa furthe-r object of the present invention to provide a three-layersolid-state image-intensifier device whose construction is very greatlysimplified.

The novel features which are believed to be characteristie of theinvention both as to its organization and method 'of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawing in which an embodiment of the invention isillustrated by way of example. It is to be expressly understood,however, that the drawing is for the purpose of illustration anddescription only and is not intended as a deiinition of the limits ofthe invention.

FIGURE l is a cross-sectional view of an embodiment constructed inaccordance with the present invention;

FIGURE 2 is a schematic lrepresentation or equivalent circuit of anelemental picture element of an image intensifying device according tothe present invention; and

FIGURE 3 is a cross-sectional view illustrating a modification of theFIGURE 1 embodiment.

Considering now the drawing, reference is made in particular to FIG. 1wherein an image intensifier in accordance with the present invention isshown to include a sandwich arrangement of a photoconductive layer 10, aferroelectric layer 11 and an elect-roluminescent layer 12, theferroelectric layer being sandwiched between the photoconductive andelectroluminescent layers. Photoconductive layer 10 may be 1 to 2 milsthick, or less, and, although photoconductive materials are well known,by way of example, it may be made of a material such as cadmium sulphideor cadmium selenide and may -be either in the powdered `or solid form.As for ferroelectric layer 11, it may be between 5 to 10 mils thick, maybe made of a material such as triglycine sulfate, Rochelle salt, bariumtit-anate, barium strontium titanate, or the like, land may be asintered layer or may be grown as a single at crystal. Finally,electroluminescent layer 12, which may be approximately 1 to 2 mils inthickness, may be any of the known electroluminescent phosphors, such ascopperactivated zinc sulphide phosphor, and may be deposited by kany ofthe known techniques such as settling or silk screening.

The free surfaces of photoconductive and electroluminescent layers 10and 12, that is to say, the top and bottom surfaces of theabove-described sandwich panel, are each covered with a transparentconducting layer, the one -overlying the phot-oconductive layer beingdesignated 13 and the other one, namely, the one covering theelectroluminescent layer, being designated 14. Transparent conductingl-ayers 13 and 14 are lm thin and may be of a material such as tin oxideor tin chloride and may be deposited by any known technique. Theselayers may also be made with thin iilms of metal, such as gold,evaporated onto their support surfaces -by the process of vacuumdeposition, the layers being thin enough to permit light to pass throughthem without diiculty.

Electrically connected between layers 13 and 14 are direct-current andalternating-current voltage sources connected in series, the D.C. sourcebeing designated 15 and the A.C. source being designated 16. As anexample of the voltages that may be involved, D.C. source 15 ispreferably volts and A.C. source 16 is preferably about 75 volts R.M.S.As shown in the figure, voltage source 15 is connected so that itsnegative terminal is connected to layer 13. As for voltage source 16,the frequency of the signal produced by this source is in the audiorange, preferably about 5,000 cycles per second. Finally, also includedin the FIG. 1 embodiment is a transparent supp-ort member or glass plate17 that supports the entire layer assembly on Ione of its surfaces. Morespecifically, as is shown in the figure, layers 10-14 rest on plate 17so that conducting layer 14 is sandwiched in between layer 12 and plate17. Transparent support members 17 may be made of a material such asPyrex glass and may be approximately 1A of an inch in thickness.

The underlying principle of the image intensifier device shown in FIG. lis presented in FIG. 2 by means of a schematic circuit which representsan elemental picture element in the intensier. In other words, theschematic circuit in FIG. 2 sets forth the electrical circuitequivalents of an elemental portion or segment of layers 10, 11 and 12.Thus, the equivalent of photoconductive layer 10` is the parallelcombination of variable resistor Rpc and capacitor Cpe, the equivalentof ferroelectric layer 11 is the parallel combination of xed resistorRf@ and variable capacitor Cie, and the equivalent of electroluminescentlayer 12 is the parallel combination of fixed resistor Rel and fixedcapacit-or Cel. These three parallel combinations, designated like thelayers they represent, namely, 10, 11 and 12, are connected in seriesbetween voltage sources 15 and 16 previously described.

In considering the operation orf this circuit, it will be assumed thatthe values of resistance are suiciently high so that thealternating-current path is to be considered as bei-ng through thecapacitors. On the other hand,

the D.-C. conductivity of the capacitors is assumed to be negligible,with the result that the direct-current path is to be considered :asbeing through the resistors. Stated otherwise, resistors Rpc, Rie andRe, may be considered as forming a voltage divider with capacitors Cpe,Cfe and Cel respectively bridged across them, the sum of the threeresistances having a much yhigher value than the sum of the threecapacitive limpe-dances. It should also be mentioned or reiterated atthis point that the resistances of the three resistors are comparable toone another in value, that is to say, of the same order of magnitude,and that this is achieved not only by appropriately choosing thethicknesses of the layers to provide the necessary -operatingconditions, but also by doping t-he -ferroelectric material. The factthat erroelectric materials can be prepared with conductivity -isindicated in the article entitled Ferroelectricity in SbSI by E.Fatuzzo, et al., published on page 2036 .in Physical Review, vol. 127,No. 6, September l5, 1962. Similar information =is available in theGerman article entitled Aufbau and IEigenschaften von PTC-Widerstandenpublished on pages 63 and 64 o-f the periodical Elektronische Rundschau,vol. 17, No. 2, 1963.

Accordingly, in operation, the effect of, radiation on thephotoconduc-tive element is to reduce its resistance, that is, theresistance of resistor Rpc, thereby reducing the voltage drop acrossthis last-mentioned resistor and correspondingly increasing the voltagedrops across resistors Rfe and Rel. This increase Iin the D.-C. voltageacross the lferroelectric element increases its A.C. impedance, that isto say, the impedance of capacitor Cfe in the equivalent circuit, sothat the A.C. current fiow through the photoconductive, .-ferr-oelectricland electroluminescent eleme-nts is correspondingly reduced.Consequently, there is a drop -in the output light from theelectroluminescent element. In other words, for the reasons give-n, the-iiow o-f A.C. current through the phosphor is therefore reduced and thelight output lowered. This is brought about by the fact that theferroelectric element has a square-.shaped hysteresis loop. As a result,it acts, essentially, as an extremely non-linear A.C. impedance, itblocking action toward the iiow of A.-C. current rising rapidly with `an-increase of field across it. Because of this, an lincrease o-.f lighton the photoconductive element, causing an increase in iield -across theferroelectric element, causes a sharp drop .in the light emitted fromthe electroluminescent element.

Similarly a reduction in input radiation will, for the reasons given,ultimately produce an increase in output light.

Hence, in etiect, a small change in input radiation on thephotoconductor produces a large change in the output light.

As we previously mentioned, the equivalent circuit in FIG. 2 is takenfor an elemental picture element in the FIG. l embodiment and, as w-illbe recognized, any such embodiment would necessarily include .a greatmany of these elemental picture elements. However, the principlesunderlying lone such picture element are equally applicable to allpicture elements. Moreover, since layers -14 are so thin, electricalcurrent iiows straight down through them and does not, to any practicalextent, iiow -or .spread sideways. Accordingly, it may be said that eachelemental picture element is isolated from or acts independently fromevery other picture element, which means that the underlying principlesof a single picture element are lalso valid [for the embodiment as awhole. Thus, a light image passing through nlm 13 and irnpinging uponphotoconductive layer 10 will result in an intensi-fied output ima-gevisible thro-ugh Iglass plate 17 and, because of the elimination of anyelectrode structure or groovi-ng technique, the resolution is greatlyimproved, being limited only by the laye-r thickness.

A modification of the embodiment presented in FIG. l

S is shown in FIG. 3 wherein like or similar parts or elements are.similarly designated. Thus, in FIG. 3, the photoconductive,ferroelectric and eleotroluminescent layers are respectively designated10i, 11 and `12. Likewise, the transparent conducting layers aredesignated 13 and 14, the D.-C. and A.C. voltage sources arerespectively designated 15 and 1-6, and the Iglass plate is designated17. With but one exception, the elements recited are mounted or-arranged as they were before and, also, are made of the same materialsand dimensions. Hence, to avoid being redundant, nothing iurther need besaid with respect to these elements. As to the exception mentioned,which constitutes the modiiication, Va thin resistive layer that isopaque to light is interposed between ferroelectric layer 11 andelectrolmninescent layer l2. This opaque resistive layer, designated.18, is preferably only a fraction of a mil in thickness and may be athin sheet of plastic, such as an epoxy resin, containing lampblack.Again, layer .1-8 may be a tine mosaic of conducti-ng elements, such asIgold, that are isolated -rom each other. Such a mosaic could bedeposited on either the rferroelectric layer or the electroluminescentlayer by evaporatin-g the mosaic material, such as the gold previouslymentioned, through a very fine lgr-id.

The yfunction of layer 18 lis to prevent or minimize light feedback,that -is to say, lits function is to eut down or reduce the amount loflight returning to the input rfrom the electroluminescent layer. Layer18 may also serve to reflect light toward Vglass faceplate 17.

Although a couple of. particular arrange-ments of the invention havebeen yillustrated above by way of example, it is not intended that `theinvention -be limited thereto. Thus, by way of example, in thearrangement of FIG. 3, opaque resistive layer 18 may, with equally goodeffeet, be -deposited between photoconductive layer 10 and Ierroelectriclayer 11 rather than between the rerroelectric layer andelectroluminescent layer 12. Aigain, ias an example, if yferroelectriclayer 11 is transparent to output light, then the positions of theelectroluminescent and -ferroelectric layers may be reversed, that is tosay, the abovesaid sandwich may be c-omposed of photoconductive,eleetroluminescent and ferroelectric layers in the order named.Accordingly, the invention should be considered to include any and allmodiii-cations, alterations or equivalent arrangements .falling withinthe scope of the annexed claims.

Having thus described the invention, what is claimed is:

1. Light amplifying apparatus comprising: continuous layers of aphotoconductive material, a ferroelectric material and anelectroluminescent mate-rial in a sandwich arrangement with thephotoconductive layer being an outside layer and in which the layers arein contact with each other throughout their `face-to-face surfaces, theparameters of said layers being such that the sum of theirdirect-current impedances is considerably ,greater than the sum of theiralternating-current impedances, said ferroelectr-ic layer being doped toreduce its directcurrent impedance to .a magnitude that is respectivelyof the same order as the direct-current impedances of thephotoconductive and electroluminescent layers; and transparentconducting layers on the top and bottom surfaces, respectively, of saidsandwich arrangement.

2. Image intensifier apparatus compris-ing: continuous layers of aphotocondructive material, .a ferroelectric material and anelectr-olluminescent material in a sandwich arrangement in which thererroelectric layer is positioned between the photoconductive andelectroluminescent layers and i-n which said layers are in contact Witheach other throughout their iace-to-face surfaces, said ferroelectriclayer being doped to reduce its resistance to a magnitude that issimilar in value to those of the photoconductive and electroluminescentlayers; continuous transparent conducting layers on the top and bottomsurfaces, respectively, orf said sandwich arrange- 7 ment; anddrect-mlrrent and alternating-current voltage 2,989,636 sourcesconnected in series between said transparent con- 3,041,490 ductinglayers. 3,054,900 3,112,404

References Cited by the Examiner UNITED STATES PATENTS 2,905,813()9/1959 Kazan Z50-213 8 Lieb Z50-213 X Rajdhman et al. Z50-213 XOfrt-huber Z50-213 Reed Z50-213 RALPH G. NILSON, Prmmy Examiner.

M. A. L-EAVITT, Assistant Examiner.

1. A LIGHT AMPLIFYING APPARATUS COMPRISING: CONTINUOUS LAYERS OF APHOTOCONDUCTIVE MATERIAL, A FERROELECTRIC MATERIAL AND ANELECTROLUMINESCENT MATERIAL IN A SANDWICH ARRANGEMENT WITH THEPHOTOCONDUCTIVE LAYER BEING AN OUTSIDE LAYER AND IN WHICH THE LAYERS AREIN CONTACT WITH EACH OTHER THROUGHOUT THEIR FACE-TO-FACE SURFACES, THEPARAMETERS OF SAID LAYERS BEING SUCH THAT THE SUM OF THEIRDIRECT-CURRENT IMPEDANCES IS CONSIDERABLY GREATER THAN THE SUM OF THEIRALTERNTING-CURRENT IMPEDANCES, SAID FERROLECTRIC LAYER BEING DOPED TOREDUCE ITS DIRECTCURRENT IMPEDANCE TO A MAGNITUDE THAT IS RESPECTIVELYOF THE SAME ORDER AS THE DIRECT-CURRENT IMPEDANCES OF THEPHOTOCONDUCTIVE AND ELECTROLUMINESCENT LAYERS; AND TRANSPARENTCONDUCTING LAYERS ON THE TOP AND BOTTOM SURFACES, RESPECTIVELY, OF SAIDSANDWICH ARRANGEMENT.